The present invention relates to a method for operating an SCR catalytic converter system of an internal combustion engine, the SCR catalytic converter system comprising at least one SCR catalytic converter and at least one upstream SCR-coated particulate filter. Furthermore, the invention relates to an SCR catalytic converter system which is set up for carrying out the method, and to a computer program, a machine-readable storage medium, and an electronic control unit which are provided for carrying out the method.
Methods and apparatuses for operating an internal combustion engine, in particular in motor vehicles, are known, in the exhaust gas region of which an SCR (Selective Catalytic Reduction) catalytic converter is arranged which reduces the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine to form nitrogen in the presence of a reducing agent. In particular, ammonia (NH3) or else, for example, formic acid can be used as reducing agent for the process of the reaction. The reducing agent or a precursor thereof is mixed into the exhaust gas upstream of the catalytic converter, as viewed in the exhaust gas direction, by, for example, NH3-separating reagents, in particular a urea/water solution, being injected. The ammonia which is released in the exhaust gas section can react with the undesired nitrogen oxides of the combustion process in the catalytic converter at a corresponding temperature. The required quantity of urea/water solution is generally dependent on the load operation of the internal combustion engine and is injected into the exhaust gas section as required.
SCR catalytic converters which are known nowadays store NH3 on the catalytic converter surface. The NOx conversion in the SCR catalytic converter is more successful, the greater the amount of reducing agent in the catalytic converter. As long as the storage capability of the SCR catalytic converter for NH3 has not yet been exhausted, excessively metered reducing agent is stored. The stored NH3 is also called the NH3 filling level. If less reducing agent is provided than is necessary for the conversion of the nitrogen oxides which are currently present in the exhaust gas, the stored reducing agent is consumed for the conversion of the nitrogen oxides and therefore the NH3 filling level is reduced.
Metering strategies which are known nowadays for SCR systems have what is known as a filling level regulating means which sets an operating point in the form of a setpoint value for the NH3 filling level in an SCR catalytic converter, a temperature-dependent setpoint filling level stipulation forming the basis, for example. Said operating point is selected in such a way that the NH3 filling level is high enough to ensure both an NOx conversion rate and a buffer for briefly occurring NOx peaks. However, the filling level should secondly also be as far as possible from the maximum storage capability, in order to avoid NH3 slip out of the system.
In order to achieve higher conversion rates in the nitrogen oxide reduction in the exhaust gas section, systems are already known which comprise two separate SCR catalytic converter devices. The supply of the SCR catalytic converter devices which are connected behind one another usually takes place in such a way that an injection position for the reducing agent is provided upstream of the first SCR catalytic converter device, as viewed in the exhaust gas direction. Part of the reducing agent which is supplied here is not consumed during the catalysis reaction in the first SCR catalytic converter device and is also not stored here, with the result that said part of the supplied reducing agent leaves the first SCR catalytic converter device again as what is known as NH3 slip. Via said NH3 slip, the second SCR catalytic converter device is supplied with reducing agent, with the result that sufficient reducing agent is also available for the second SCR catalytic converter device, as described, for example, in German laid-open specification DE 10 2011 085 952 A1. Systems of this type having two SCR devices can be controlled for process management in a conventional way by way of double software, it being possible for the variables for the process management for each SCR device to be taken from catalytic converter models which are known per se. Models of SCR catalytic converters can be implemented in the control units of modern motor vehicles and depict both the NOx conversion of the SCR catalytic converter and the NH3 slip. In order to improve the double modelling which is comparatively complicated and susceptible to faults, German laid-open specification DE 10 2012 221 905 A1 describes a method for operating an SCR catalytic converter system with two SCR devices, in which method the metering of the reducing agent is set upstream of the first SCR catalytic converter on the basis of model-based pilot control in a manner which is dependent on a desired overall degree of efficiency.
German laid-open specification DE 10 2013 217 169 A1 describes a method and a system for exhaust gas aftertreatment, the system having an oxidation catalytic converter, a first SCR catalytic converter device and a second SCR catalytic converter device in the direction of the exhaust gas flow. An injector for introducing reducing agent is situated immediately upstream of the first SCR catalytic converter device. One or more further injectors can optionally also be provided along the exhaust gas section, a further injector being situated, for example, between the first SCR catalytic converter device and the second SCR catalytic converter device. A method for operating said exhaust gas aftertreatment system is described, in which method a multiple of the instantaneously required metering of the reducing agent is injected intermittently.